[Caml-list] Porting old-style ppx to driver-style?

["Paul A. Steckler" <steck@stecksoft.com>]

There's a nice OCaml library, available via OPAM as "lens", which
allows deriving lenses for record types.

By adding the following clause in a jbuild file, the ppx works fine:

  (preprocess (action (run ppxfind -legacy
lens.ppx_deriving,ppx_deriving.std --as-pp ${<})))

But adding "ppx_inline_test" to this clause doesn't allow an
"(inline_tests)" clause to work, and when building a library, only one
"preprocess" clause appears to have an effect.

Using the ppx within a "(preprocess (pps ...))" clause does not
succeed. So I can have lenses, or inline tests, but not both.

I believe the solution is to turn the ppx into a new-style "driver".
What's the best way to do that? Is it as simple as changing the build
to use a jbuild as described in the section "Creating a new Ppx_driver
based rewriter"?:

  https://github.com/janestreet/ppx_driver

Do I need to make any other code changes?

-- Paul

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